M. C. A. M. Peters, F. M. Braal, C. H. L. Limpens, E. van Bokhorst
INSTALLATION EFFECTS ON VORTEX FLOWMETERS - THE IMPACT OF PIPING AND FLOW DYNAMICS ON THE SENSOR SIGNAL
At the Flow Centre of the TNO Institute of Applied Physics installation effects on various types of flowmeters are being studied. Recently, an extensive study on the impact of pulsations and mechanical pipe vibrations on the output of five different make 3-inch industrial vortex flowmeters has been finalised and the comparison has been presented at several occasions [1,2]. To study the impact of bluff body dimensions, the experiments have been repeated for a 1.5” and 4” vortex flowmeter. Also a detailed analysis has been carried out to study the impact of unsteady flow and pipe vibrations on the sensor signal.
The experiments have been conducted in the TNO Flow Centre air rig over a range of pulsation and vibration frequencies and amplitudes at a mean flow from 0-700 m³/h, covering the full range of the 2 and 3-inch vortex flowmeters. The pulsation tests have been conducted with sinusoidal flow pulsations in the range from 20-400 Hz and flow pulsation amplitudes from 1 to 30 % rms of the average flow. The sensor signal shows a number of frequencies, which can be related to the frequency and amplitude of the pulsating flow. Errors in reading are not only caused by the lock-in effect, i.e. when the vortex shedding frequency is coupled with the pulsation frequency, but also when due to the distorted sensor signal an irregular pulse signal is generated from the Schmitt trigger. Also for the 1.5” and 4” vortex flowmeter, large systematic errors occur due to the lock-in effect if the vortex frequency of the meter comes close to the frequency of the pulsating flow. Lock-in sometimes occurs already at very low pulsation amplitudes of 3 % rms. Lock-in is shown to occur not only if the vortex flowmeter frequency f v meets the pulsation frequency fp, but in several cases also if fv = 0.25, 0.5, 1.0 , 1.5, 2.0 times fp .
The impact of mechanical pipe vibrations imposed on a flowmeter under flow conditions is completely different from the effect of pulsating flow. In the case of pipe vibrations, the vortex shedding process is not influenced by the vibrations, but the sensor cannot distinguish between vortex shedding signal and pipe wall vibration. The effect on the sensor signal can lead to large systematic errors which depend on vibration frequency and amplitude. The location, type of sensor and the type of filter are important parameters, by which further improvement in reducing the sensitivity for pipe wall vibrations can be achieved.
The application of digital signal processing techniques for vortex flowmeters can improve the accuracy of vortex flowmeters in more critical conditions like pulsating flow and pipe vibrations, which are likely to occur in many applications in the process industry.
